1. Field of the Invention
The subject disclosure relates to a fuel control system for use with gas turbine engines, and more particularly to, a fuel control system which utilizes computed signals from an embedded, real-time thermodynamic engine model when attempting to match actual core engine acceleration or deceleration rates to the demanded rate.
2. Background of the Related Art
Typically, a gas turbine engine control system modulates fuel flow to the engine in order to match or xe2x80x9ctrackxe2x80x9d the actual rate of change of the gas generator speed (NDOTActual) to the demanded rate of change of the gas generator speed, NDOTDemand. The maximum demanded NDOT rate is obtained from an acceleration schedule. The acceleration schedule is traditionally provided by the engine manufacturer and is developed over time to protect the engine from surge, stall and overtemperature. As a result, the acceleration schedule is specific or unique to a particular engine model. The schedule typically represents NDOTDemand as a function of measured gas generator speed (NH) and inlet air temperature and pressure. The schedule is not linear, but of complex shape. The complexity of the schedule is partly due to the need to prevent the engine from operating in the compressor stall region.
State-of-the-art digital control systems typically use a proportional plus integral (and sometimes derivative) NDOT control loop to modulate fuel flow and null out the error between the measured actual acceleration/deceleration rate of the core engine gas generator (NDOTActual) and the demanded rate (NDOTDemand). Since the engine is a highly non-linear complex machine, the matching or tracking of actual versus demanded NDOT rate is sometimes imperfect, especially during rapid engine accelerations or decelerations. More specifically, during severe operational transients, the control system is unable to drive the error between NDOTDemand and NDOTActual to zero.
The inability to track the NDOTActual rate to the NDOTDemand rate is partly caused by control design tradeoffs, namely bandwidth limitations which result from an overriding desire to insure control loop stability. More importantly however, current state-of-the-art control systems do not account for external disturbances to the NDOT control loop, such as real-time thermodynamic engine effects, which adversely affect NDOT rate tracking.
As a result of the inability to accurately track the actual NDOT rate to the demanded rate, engine surge events could occur if actual NDOT overshoots the acceleration limit. An engine surge creates a sudden torque disturbance to the driven load. In a helicopter application, an engine surge event typically imparts a torque disturbance to the load system, which consists generally of an engine output shaft, a clutch, a gearbox, and shaft driven main and tail rotors. The sudden torque disturbance can cause the underdamped rotor drive train to ring which can result in transient overstressing of mechanical parts and result in engine drive train damage.
Therefore, there is a need for an improved NDOT tracking system which during operational transients, more accurately matches the NDOTActual rate to the NDOTDemand rate by accounting for real-time thermodynamic engine effects.
The subject disclosure relates to fuel control systems which account for real-time thermodynamic engine effects when attempting to match or track the NDOTActual rate to the NDOTDemand rate. The fuel control systems disclosed herein recognize that a significant cause of poor NDOT rate tracking is the effect of heat being transferred between the combustion gases and the engine metal. During an engine acceleration, heat is diverted from the burned fuel being metered by the NDOT control to the engine metal, resulting in a reduced actual NDOT rate and thereby degrading NDOT tracking performance. Conversely, during an engine deceleration, heat is transferred from the engine metal to the combustion gases, resulting in an increase in NDOTactual and also degraded tracking performance.
The subject disclosure is directed to a fuel control system for use with a gas turbine engine which includes a mechanism for measuring several engine operating parameters and a mechanism for determining an initial engine fuel demand based on the measured engine operating parameters. The control system further includes a mechanism for estimating, during engine operation and based on the measured operating parameters, the amount of heat transferred between the fuel combustion gases and the engine metal and estimating an effective fuel flow adjustment therefrom. The control system disclosed herein also includes a mechanism for determining a final engine fuel demand based on the initial predicted engine fuel demand and the estimated effective fuel flow adjustment.
Preferably, the mechanism for measuring a variety of engine operating parameters includes a device which provides a signal indicative of the actual rotary speed of the engine gas generator and an element for measuring the actual engine compressor discharge pressure.
It is presently envisioned that the mechanism for determining the initial engine fuel demand further includes a closed loop NDOT controller that modulates fuel flow in response to a comparison of the actual rate of change of gas generator speed, determined from the gas generator speed signal, to a maximum and minimum desired rate of change of gas generator speed. It also envisioned that the maximum and minimum desired rate of change of gas generator speed is determined based on acceleration and deceleration schedules and is a function of the gas generator speed signal and inlet air temperature and pressure.
Preferably, the mechanism for estimating the amount of heat transferred between the fuel combustion gases and the engine metal includes an engine combustion model. The combuster model estimates the amount of heat generated by fuel combustion, the amount of heat generated by supply air compression, and the gas generator exit gas temperature.
The mechanism for estimating the effective fuel flow adjustment preferably includes a fuel flow adjuster model. The fuel flow adjuster model predicts the effective fuel flow adjustment required to account for the real-time thermodynamic effects from the estimated heat transferred, the gas generator efficiency and the heating coefficient of fuel. In a preferred embodiment, the mechanism for estimating the effective fuel flow adjustment further comprises an amplifier means for providing a gain amplified effective fuel flow adjustment.
The fuel control system disclosed herein also preferably includes a fuel metering system which supplies fuel to the engine based on the final predicted engine fuel demand. The fuel metering device can include a fixed displacement pump and metering/pressure regulating valves or be a variable delivery system.
The subject disclosure is also directed to a method of fuel control for gas turbine engines having a compressor and a gas generator. The fuel control method disclosed herein includes the steps of measuring a plurality of engine operating parameters and determining an initial engine fuel therefrom. The method of fuel control also includes the steps of estimating during engine operation and based on the plurality of measured operating parameters, an amount of heat transferred between fuel combustion gases and engine metal, estimating an effective fuel flow adjustment based on the estimated heat transfer between the combustion gases and the engine metal, and determining a final engine fuel demand based on the initial engine fuel demand and the estimated effective fuel flow adjustment.
Preferably, the steps of measuring a variety of engine operating parameters includes the steps of measuring the actual gas generator speed, providing a signal indicative thereof, measuring actual engine compressor discharge pressure, and providing a signal indicative thereof.
It is envisioned that the step of determining the initial engine fuel demand includes the use of a fuel flow controller which iteratively compares an actual rate of change of gas generator speed, determined from the gas generator speed signal, to a desired rate of change of gas generator speed. The desired rate of change of gas generator speed is determined from an acceleration/decelleration schedule and is a function of the gas generator speed signal.
It is presently preferred that the step of estimating the amount of heat transferred between the fuel combustion gases and the engine metal includes the steps of estimating an amount of heat generated by the fuel combustion, estimating an amount of heat generated by supply air compression, and estimating the gas generator exit gas temperature.
The step of estimating the effective fuel flow adjustment preferably includes determining the effective fuel flow adjustment from the estimated heat transfer, gas generator efficiency and a heating coefficient of fuel. The step of estimating the effective fuel flow adjustment further includes an amplifier for providing a gain multiplied effective fuel flow adjustment and a signal indicative thereof.
Preferably, the method of fuel control further includes supplying, by means of a fuel metering system, fuel to the engine based on the signal of final engine fuel demand. In one embodiment, the fuel metering device includes a variable displacement vane pump.
The subject disclosure is also directed to a fuel control system for use with a gas turbine engine which includes a means for measuring a plurality of engine operating parameters, a means for determining an initial engine fuel demand based on the plurality of measured engine operating parameters. The fuel control system further includes a means for measuring during engine operation an amount of heat transferred between the fuel combustion gases and the engine metal, a means for estimating an effective fuel flow adjustment based on the measured heat transfer, and a means for determining a final engine fuel demand based on the initial engine fuel demand and the estimated effective fuel flow adjustment.
The subject disclosure is also directed to a method of fuel control for gas turbines which includes the steps of measuring a plurality of engine operating parameters, determining an initial engine fuel demand based on the plurality of measured engine operating parameters, and measuring an amount of heat transferred between fuel combustion gases and engine metal. The method further including estimating an effective fuel flow adjustment based on the measured heat transfer between the combustion gases and the engine metal and determining a final engine fuel demand based on the initial engine fuel demand and the estimated effective fuel flow adjustment.
Those skilled in the art will readily appreciate that the subject invention more accurately matches the NDOTActual rate to the NDOTDemand rate of the gas turbine engine by accounting for real-time thermodynamic engine effects. These and other unique features of the fuel control system disclosed herein will become more readily apparent from the following description, the accompanying drawings and the appended claims.